1. Field of the Invention
The present invention relates to a photographing apparatus having a function capable of automatically focusing by detecting a focus position using auto-focus (AF) of a charge coupled device (CCD), namely, CCD-AF, for example.
2. Description of the Prior Art
A conventional photographing apparatus is, for example, a video camera, a digital camera, or the like. A conventional video camera employs a contrast system in order to perform auto-focusing. In this contrast system, with a focusing lens being driven, contrasts of photographed images acquired at respective driving steps are obtained as evaluation values, and a lens position having the highest evaluation value among them is set as a focus position.
In this video camera, a CCD photographing device used has pixels in the order of several hundreds of thousands because it is used to photograph moving images. For this reason, high accuracy in AF is not required. If a focusing speed is high too when photographing video, movement of focus will be frequently carried out in accordance with the movements of the camera and a subject, and hence user's eyes will not be able to follow the focus movement, thus causing the user to feel the abnormal of images.
In contrast, the digital cameras for photographing still-frame images are required to quickly perform auto-focusing and not to miss a shutter chance.
However, in digital cameras having a CCD photographing device with a high pixel density, because a focus position must be figured out with high accuracy, a lens position with the highest evaluation value is needed to be detected by repeating driving of a lens by a fine pitch based on the depth of field.
Thus, in the digital cameras using the CCD photographing device having high-density, the use of conventional contrast-system auto-focusing increases the driving number of lens, thus taking a long time to figure out a focus position, leading to miss a shutter chance.
Particularly, in cases where a subject is extremely out of focus, it will take a very long time to obtain the focus position, and hence it is necessary to efficiently perform auto-focusing so as not to miss the shutter chance.
In view of the above-mentioned problems of the contrast system employed in such video cameras, a digital camera that permits effectively obtaining a focus position is proposed (see, for example, Japanese Patent Laid-Open Publication No. 2001-281529).
In this digital camera, a driving circuit drives a photographing lens in several steps by a larger distance than the depth of field, figures out an evaluation value based on a photographed image obtained from the photographing device in each driving step, and then performs predetermined interpolation of a plurality of the evaluation values of plural driving steps, thereby obtaining a focus position of the photographing lens for causing a focal plane to coincide with a photographing plane. This driving circuit drives the photographing lens to the focus position based on focus position data obtained.
Thus, the conventional digital camera achieves reduction in time to obtain the focus position, while keeping high accuracy by the interpolation of the evaluation values.
However, since this camera drives the photographing lens by a larger interval or pitch than the depth of field, when dividing a scanning range for auto-focusing into intervals each being larger than the depth field, the appropriate accuracy of the interpolating operation cannot be obtained, or the interpolation cannot be performed depending on focal length and brightness of a lens (F number).
FIG. 13(a) shows a state of one example having the above-mentioned problem. In FIG. 13(a), a horizontal axis indicates a position of a photographing lens based on the reciprocal of a focal length (in detail, Lim (∞) is referred to as a focal-length infinite distance, and Lim (Nr) is referred to as a focal-length closest position), while a vertical axis indicates a focusing evaluation value at each photographing lens position.
This curve of evaluation values has a feature that an intersection point of the horizontal axis and a line vertical to the horizontal axis passing through a maximum point of the evaluation value corresponds to a focus position.
FIG. 14 shows a general example.
In the case as shown in FIG. 13(a), the depth of field 50a is so large that there are few points for sampling, resulting in a low accuracy of interpolation.
As shown in FIG. 13(b), when a curved line of evaluation values is geometrically unsymmetrical with respect to a focal length of a photographing lens, highly accurate interpolation cannot be provided.
Further, as shown in FIGS. 13(c) and (d), in cases where a focus position is at an infinite distance, or at the closest point or its vicinity, a steep extension method used in the prior art cannot be employed, resulting in failing to conduct interpolation.